1. Field of the Invention
This invention relates generally to direct oxidation fuel cells, and more particularly, to fuel formulations for such fuel cells.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and generally requires expensive components, which occupy significant volume, reformer-based systems are presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. In direct oxidation fuel cells of interest here, a carbonaceous fuel (including, but not limited to, liquid methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised of predominantly methanol or methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate, as is discussed further hereinafter.
Typical DMFC systems include a fuel source, fluid and effluent management systems, and air management systems, as well as a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection, fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system generally take place within the MEA. In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the membrane electrolyte, which is non-conductive to the electrons. The electrons travel through an external circuit, which connects the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
A typical MEA includes an anode catalyst layer and a cathode catalyst layer sandwiching a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”, sometimes also referred to herein as “the catalyzed membrane”). One example of a commercially available PCM is NAFION® (NAFION® a registered trademark of E.I. Dupont de Nemours and Company), a cation exchange membrane based on polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. A PCM that is optimal for fuel cell applications possesses a good protonic conductivity and is well-hydrated. On either face of the catalyst coated PCM, the MEA typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid or gaseous fuel over the catalyzed anode face of the PCM, while allowing the reaction products, typically gaseous carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen to the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM through the load.
Direct oxidation fuel cell systems for portable electronic devices ideally are as small as possible for a given electrical power and energy requirement. The power output is governed by the reaction rates that occur at the anode and the cathode of the fuel cell operated at a given cell voltage. More specifically, the anode process in direct methanol fuel cells, which use acid electrolyte membranes including polyperflourosulfonic acid and other polymeric electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, water molecules are consumed to complete the oxidation of methanol to a final CO2 product in a six-electron process, according to the following electrochemical equation:CH3OH+H2OCO2+6H++6e−  (1)
Since water is a reactant in this anodic process at a molecular ratio of 1:1 (water:methanol), the supply of water, together with methanol to the anode at an appropriate weight (or volume) ratio is critical for sustaining this process in the cell. In fact, in typical DMFC systems the water:methanol molecular ratio in the anode of the DMFC has to significantly exceed the stoichiometric 1:1 ratio suggested by process (1), based on the prior art of direct methanol fuel cell technology. This excess is required to guarantee complete anodic oxidation to CO2, rather than partial oxidation to either formic acid, or formaldehyde, 4e− and 2e− processes, respectively, described by equations (2) and (3) below:CH3OH+H2OHCOOH+4H++4e−  (2)CH3OHH2CO+2H++2e−  (3)
In other words, equations (2) and (3) are partial processes that are not desirable and which might occur if the balance of water and methanol is not maintained correctly during a steady state operation of the cell. Particularly, as is indicated in process (3), which involves the partial oxidation of methanol, water is not required for this anode process and thus, this process may dominate when the water level in the anode drops below a certain point. The consequence of process (3) domination, is an effective drop in methanol energy content by about 66% compared with consumption of methanol by process (1), which indicates a lower cell electrical energy output. In addition, it could lead to the generation of undesirable anode products such as formaldehyde.
Several techniques have been described for providing an effective methanol/water mixture at the anode catalyst in a DMFC. Some systems include feeding the anode with a very dilute methanol solution and actively circulating water found at the cathode back to the cell anode and dousing recirculated liquid with neat methanol stored in a reservoir. Other systems are passive systems that require no pumping and which can carry a high concentration of fuel. Some of these include recirculation of water; however, other systems have been described in which water does not need to be recirculated from the cathode because water is pushed back from the cathode through the membrane to the anode aspect.
One example of a non-recirculating system is described in commonly-assigned U.S. patent application Ser. No. 10/413,983, filed on Apr. 15, 2003, by Ren et al. for a DIRECT OXIDATION FUEL CELL OPERATING WITH DIRECT FEED OF CONCENTRATED FUEL UNDER PASSIVE WATER MANAGEMENT, which is incorporated herein by reference, and U.S. patent application Ser. No. 10/454,211, which was filed on Jun. 4, 2003 by Ren et al. for PASSIVE WATER MANAGEMENT TECHNIQUES IN DIRECT METHANOL FUEL CELLS, which is also incorporated herein by reference.
Some of these techniques may incorporate a vaporous fuel being delivered to the anode aspect for the reactions. In the case of delivering a vaporous fuel, the above-cited patent applications describe providing a pervaporation membrane that effects a phase change from a liquid feed fuel to a vaporous fuel, which is then delivered to the anode aspect, as a vapor.
As noted, the fuel cells operating with vaporous fuels typically include the above-mentioned pervaporation membrane, which effects the phase change from liquid to vapor prior to the fuel being delivered to the anode aspect of the fuel cell. However, such pervaporation membranes may need to be specially engineered, which can be costly. In addition, some such membranes, though useful for delivering the vaporous fuel, can, degrade in the presence of the methanol fuel, compromising the delivery of fuel.
Some systems that use a liquid fuel require additional circulation systems, including, pumps, valves and other fluid management components and control systems to deliver the fuel at a controlled rate and in the desired manner. This typically requires additional components that consume power, increasing the parasitic losses within the system, and adding additional complexity, expense, and volume. In handheld electronic devices and other portable electronic devices, form factors are critical and a premium is placed on the ability of a power source to fit within the designated form factors. Additional recirculation subsystems, including pumps, valves and other fluid management equipment and components may increase the size of the overall fuel cell system, and in some cases may increase it significantly. A liquid fuel can also require a more complex fuel delivery system that may include an expansion bladder, which, when compressed, expresses the fuel in a controlled manner. One example of such a system is described in U.S. patent application Ser. No. 10/041,301, filed on Jan. 8, 2002, by Becerra et al. for a Fuel Container and Delivery Apparatus for a Liquid Feed Fuel Cell System, which is incorporated herein by reference. However, even though such expansion bladders and optional force-applying elements may be desirable in some instances, in other instances they can increase the volume, complexity and weight of the fuel delivery cartridge.
Some of the disadvantages with certain presently existing liquid fuel storage and delivery subsystems can be addressed using a vapor fed system. For example, the systems such as that described in the above-cited commonly owned U.S. patent application Ser. No. 10/413,983, which has been incorporated herein by reference, use a liquid fuel which then undergoes a phase change when passing through a pervaporation membrane, and thus such systems still may need to carry liquid fuel in a storage tank or other container. Also, this liquid fuel may have a tendency to flow within the container undesirably as the orientation of the container changes during use, which may tend to reduce the fuel efficiency to the anode.
Another issue that arises with respect to usage of carbonaceous fuel, such as methanol, in a consumer electronic device, is that of maintaining the integrity of the cartridge so that there is no leakage of the fuel. For example, when using a liquid fuel, a crack in the fuel cartridge may result in the fuel leaking out of the cartridge. Sometimes additives are employed within a container to cause the fuel to be more recognizable. Safe disposal of fuel cartridges after the fuel supply is exhausted is also a consideration with respect to consumer use of direct oxidation fuel cells.
Some of the disadvantages with certain liquid fuel and vapor fed system can be addressed using a gel-based fuel substance and related system, such as that described in commonly-assigned U.S. patent application Ser. No. 10/688,433, filed on Oct. 17, 2003, by Becerra et al. for a FUEL SUBSTANCE AND ASSOCIATED CARTRIDGE FOR FUEL CELL, which is incorporated herein by reference. In such systems, however, depending on the operating conditions there may be lower feed rates and fuel extraction efficiencies than desired, in some cases, especially in low temperature environments or where the fuel is exposed to significant vibration or shock.
Therefore, there remains a need for a fuel container, and an associated fuel formulation in which the fuel is a freely flowing liquid, yet controlled against substantial undesirable flow within the container, has orientation independence, and maintains a high feed rate and high fuel extraction efficiency with less crusting and less affectivity to vibration and shock. It is also an object of the invention to provide a safe, easy to handle and low-cost fuel container and associated fuel formulation for use with direct oxidation fuel cells that may be readily employed in consumer electronic devices.